The preferred embodiment relates to an evacuable flat panel solar collector comprising at least one absorber, at least one conduit, a holding structure and at least one transparent wall. The preferred embodiment further relates to a flat panel solar collector system which comprises at least one flat panel solar collector according to the preferred embodiment and at least one mirror, and also to a flat panel solar collector array. Finally, the preferred embodiment relates to a method for the preparation of an evacuable flat panel solar collector according to the preferred embodiment.
Solar collectors, in particular flat panel solar collectors, are well-known devices which are usually used to absorb and transfer solar energy into a collection fluid. Principally, solar collectors are comprised of a blackened absorbing cylinder or plate contained in a housing which is frontally closed by a transparent window pane. Due to the diluted nature of solar light, in order to increase the operating temperature by reducing the thermal losses, solar collectors may be evacuated during use to eliminate gas convection and molecular conduction. Very high temperatures could also be achieved by light focusing. However, only direct light may be focusing, while diffuse light is lost. Therefore, this solution is not very attractive for regions, like central Europe, where about 50% of the solar light is diffuse. As the evacuation of flat panel solar collectors is problematic due to the need of a structure which is able to maintain high vacuum even under the huge forces resulting from atmospheric pressure, the focus has been on solar collectors which are based on a cylindrical glass envelope containing a cylindrical or flat absorber. Such a design can, for example, be found in U.S. Pat. No. 4,002,160 where a multiple tube solar energy collector having a diffusely reflecting surface positioned behind a collector tube array is disclosed which includes a plurality of double-wall tubular members wherein the outer wall is made of a glass material which is transparent about its entire circumference.
According to U.S. Pat. No. 4,579,107 a tubular collector is disclosed having very advantageous characteristics which is achieved by a method which is used to create both solar selective surfaces or coatings and reflective surfaces or coatings on glass by depositing, by spraying, molten metal onto their respective surfaces so that glass fuses upon contact with the molten metal, resulting in good adhesion and thermal contact.
Also the solar energy collection system according to U.S. Pat. No. 3,960,136 relies on the use of a double-wall glass tube the outer wall thereof being transparent about substantially its entire circumference. The space between the double walls is sealed at a sub-atmospheric pressure.
Although it has been well-known that versus an array of tubular collectors, as for example disclosed in U.S. Pat. No. 4,002,160, flat panel collectors allow for a maximum availability of energy for absorption, tubular solar collectors are still most often considered to be advantageous due to their easier manufacturing of the glass-to-metal seal, as required for maintaining high vacuum.
In U.S. Pat. No. 4,084,576 a bulb-type solar energy collector is disclosed which comprises a blackened solar absorber which is inserted into a flat lamp envelope thereby making use of a reliable sealing technique which is known for example from TV cathodic tubes.
From U.S. Pat. No. 3,916,871 a flat panel solar collector module can be derived which comprises a housing with an evacuated chamber defined therein, a transparent planar wall forming one side of said chamber and a radiant energy absorber with flow passages therein which is thermally insulated from the housing. In one embodiment a vacuum pump is connected through suitable conduits to the collector module to evacuate the same from time to time as necessary. According to U.S. Pat. No. 3,916,871 a vacuum in the area of one Torr (1 mm Hg) is considered to be sufficient to eliminate convection losses. However, in this document it is admitted that very low pressures that essentially eliminate also conduction losses would require a technology not commercially available.
In U.S. Pat. No. 5,653,222 a flat panel solar collector is disclosed with which it is attempted to provide an evacuated flat panel collector having structures sufficient to resist the forces applied by the atmosphere to an evacuated envelope. Heat losses from the absorber plate due to convection, conduction and thermal infra-red emissions, commonly called radiation, shall be overcome by a flat panel solar collector comprising a rear housing which is configured to provide a series of parallel cells, preferably semi-circular in cross-section, whereby each such cell is adapted to support the primary glazing and to receive a fin-tube absorber. These fin-tubes occupy at least 90% of the open area between the side walls of the cells such that the majority of radiation is absorbed and little radiation passes between the fin-tube absorbers and the side walls. It is stated that the circular cross-section of individual cells provides the best resistance to the deformation forces of the internal vacuum. A flat panel solar collector according to U.S. Pat. No. 5,653,222 affords a multitude of components the dimensions of which have to be accurately determined and which also have to be arranged in a complex predetermined fashion. Accordingly, flat panel solar collectors based on U.S. Pat. No. 5,653,222 are rather expensive and are also rather difficult to manufacture.
In U.S. Pat. No. 4,455,998 use is made of a solar collector which is comprised of at least one sealed evacuated transparent tube or envelope containing a heatable, reversible hydrogen getter comprised of one or more of the metals titanium, zirconium, hafnium, scandium, yttrium, lanthanum, the rare earths, strontium, barium, vanadium, niobium, tantalum, thorium and alloys thereof in a partly hydrogenized condition. The hydrogen pressure is increased by heating the reversible hydrogen getter which then releases hydrogen, while hydrogen is taken up again when the reversible hydrogen getter cools down. This mechanism ensures that the solar collector maintains its normal high efficiency as the losses of the solar collector can be increased by increasing the hydrogen pressure when the heat production of the solar collector exceeds the storage capacity of the remainder of the installation so that the temperature of the absorber tends to become too high. As an envelope for the solar collector only a glass tube is disclosed which has a round cross-section and encloses a plate-shaped absorber which is thermally conductively connected to the evaporator section of a heat pipe.
In spite of the merits of the solar collectors so far designed, the evacuated tubular collectors still present some major inconveniences. Each tube requires a glass-to-metal seal at each end with bellows to reduce heat conduction and to compensate for differential thermal expansion of the cooling pipe with respect to the room temperature envelope. Further, the tubes must be spaced apart to avoid shadowing, which results in a loss of absorbing capacity. Also, the maintenance and cleaning of multi-tube structures is rather problematic. Therefore, quite often an additional front glass is added to alleviate this problem, however resulting in an additional loss of transmitted light.
Although the above disadvantages of tubular solar collectors can at least partially be overcome by a flat evacuated solar panel, the major drawback of the flat systems still is that a large flat surface is less adequate to withstand atmospheric pressure. In addition, the peripheral glass-to-metal seal still gives cause for major problems. Probably because of these difficulties, the flat solar collector of U.S. Pat. No. 3,916,871 relies on a housing made from plastics having also a transparent plastic front. As a consequence this flat solar collector is evacuated only to 1 Torr, a pressure which may be sufficient to eliminate air convection, but not molecular conduction.